Nonvolatile semiconductor memory device

ABSTRACT

A memory cell and a selection transistor for selecting the memory cell are provided. The memory cell includes a floating gate formed on a semiconductor substrate via a first gate insulation film, a pair of first diffusion layers positioned on the opposite sides of the floating gate and formed in the substrate, first and second control gates formed on the opposite sides of the floating gate to drive the floating gate, and an inter-gate insulation film formed between the first and second control gates and the floating gate. The selection transistor includes a selection gate•wiring including a first portion constituted of the same conductive layer as the first conductive layer, and a second portion constituted of the same conductive layer as the second conductive layer, and a second diffusion layer formed in the substrate, facing the second portion of the selection gate•wiring.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Applications No. 2003-194396, filed Jul. 9, 2003;and No. 2004-189817, filed Jun. 28, 2004, the entire contents of both ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a nonvolatile semiconductor memory devicehaving a multilayer gate structure including a floating gate and acontrol gate.

2. Description of the Related Art

FIGS. 1 through 3 illustrates a known NAND type EEPROM realized byutilizing shallow trench isolation (STI). FIG. 1 is a schematic planview and FIGS. 2 and 3 are two different cross-sectional views of FIG.1.

As shown in FIG. 2, a gate insulation film GI, which is a tunnelinginsulation film, is formed on a silicon substrate (Si-sub) and floatinggates FG are formed thereon. The floating gates FG of adjacent cells areseparated and electrically insulated from each other. The structure thatseparates adjacently located floating gates FG apart from each other isreferred to as a slit. The floating gates FG between a pair of slits arecovered at the top and the opposite lateral sides by an inter-gateinsulation film IGI. Each floating gate FG can be made to hold anelectric charge for a long period because it is covered by a tunnelinginsulation film and an inter-gate insulation film.

A control gate CG is formed on the inter-gate insulation film. Normally,a control gate CG is shared by a large number of cell transistors andadapted to drive the number of cell transistors simultaneously. Thecontrol gate CG is also referred to as word line WL.

On the other hand, the cross-sectional view of FIG. 3 is taken along abit line BL. Stacked gate structures illustrated in FIG. 3 are arrangedon the substrate in rows along the direction of bit lines BL. Each celltransistor is processed in a self-aligning manner by means of resist ora processing mask layer. In the NAND type memory device where a numberof cells are connected in series by way of select gates, adjacent cellsshare a source and a drain in order to reduce the area occupied by eachcell. Each word line WL and the gap separating adjacent word lines WLare formed with minimum feature size by micro-processing.

Electrons are injected into a floating gate FG by applying a high writepotential to the corresponding control gate CG and grounding thesubstrate. As cell transistors are micronized, an increased parasiticcapacitance appears between adjacent cells and between a floating gateFG and a peripheral structure. For this reason, there is a tendency ofraising the write voltage of cell transistors for the purpose ofincreasing the data writing rate. Control gates CG need to be reliablyinsulated from each other and word line drive circuits are required towithstand high voltages when a high voltage is used for the writevoltage. This poses a problem when arranging memory elements at highdensity and driving them to operate at high speed.

It is possible to roughly estimate the potential required for writeoperation by seeing the structure shown in FIGS. 1 and 3. The controlgate CG and the floating gate FG and the floating gate FG and thesubstrate can be regarded as capacitors where the gate insulation filmand the tunneling insulation film are respectively sandwiched. In otherwords, as seen from the control gate CG, the memory cell is equivalentto a structure where two capacitors are connected in series.

FIG. 4 is an equivalent circuit diagram of a cell that is obtained whenthe capacitance of the capacitor between the control gate CG and thefloating gate FG is Cip and the capacitance of the capacitor between thefloating gate FG and the substrate is Ctox. The electric potential Vfgof the floating gate FG when a high write potential (Vpgm=Vcg) isapplied to the control gate CG is defined by Cip and Ctox and can beroughly estimated by using the formula below:Vfg=Cr×(Vcg−Vt+Vt 0),where Cr=Cip/(Cip+Ctox) and Vt represents the threshold voltage of thecell transistor while Vt0 represents the threshold voltage (neutralthreshold voltage) when the floating gate FG is totally free fromelectric charge.

The higher the electric potential Vfg of the floating gate FG, thestronger the electric field applied to the tunneling insulation film soinjection of electrons into the floating gate FG can easily take place.It will be appreciated from the above formula that the value of Vfg canbe raised by increasing the capacitance ratio (Cr) provided that Vcg isheld to a constant level. In other words, it is necessary to make Ciphave a large value relative to Ctox in order to reduce the writevoltage.

The capacitance of a capacitor is proportional to the dielectricconstant of the thin film arranged between the electrodes and the areaof the opposed electrodes and inversely proportional to the distancebetween the opposed electrodes. A write/erase operation is obstructedwhen a leak current flows through the tunneling insulation film forallowing an electric charge to pass through for the purpose of thewrite/erase operation. Therefore, a technique of increasing the contactarea of the gate insulation film and the floating gate FG and that ofthe gate insulation film and the control gate CG is normally used toincrease the value of Cip. Techniques such as increasing the top surfaceof the floating gate FG by reducing the width of the slit (dimension 1 ain FIG. 2) and increasing the length of the lateral walls of thefloating gate FG (dimension 1 b in FIG. 2) by increasing the filmthickness of the floating gate FG have been developed to date.

However, when such a technique is used, the slit needs to be extremelymicronized relative to the dimensions of the gate and the wiringmaterials and the difficulty of forming the gate increases as thefloating gate FG is made thicker. Additionally, the parasiticcapacitance between FG—FG increases as a result of micronization. Inshort, it obstructs micronization of cell transistors to maintain thecapacitance ratio.

It is conceivable to reduce the write voltage by modifying theconfiguration of the floating gate FG and the control gate CG.

As a matter of fact, Japanese Laid-Open Patent (Kokai) No. 11-145429describes a NAND type EEPROM that is designed to allow write/erase/readoperations to be performed with a low voltage by increasing thecapacitance between booster plates.

Japanese Laid-Open Patent (Kokai) No. 2002-217318 describes anonvolatile memory device including micronized elements that arerealized by raising the coupling ratio of the floating gate and thecontrol gate and thereby reducing the write voltage.

Japanese Laid-Open Patent (Kokai) No. 2002-50703 describes a nonvolatilesemiconductor memory device including MOSFETs that show improvedwrite/erase/read characteristics and area realized by forming floatinggate at opposite lateral sides of each control gate.

Furthermore, Y. Sasago et al. “10-MB/s Multi-Level Programming ofGb-Scale Flash Memory Enabled by New AG-AND Cell Technology” 2002 IEEEIEDM, pp. 952–954 describes an AG-AND memory cell where an assist gateis arranged adjacent to a floating gate.

However, it is still difficult to increase the capacitance between thecontrol gate and the floating gate by means of the above described priorart. In other words, it is difficult to reduce the write voltage andrealize a highly integrated memory that operates at high speed by meansof the prior art. Therefore, nonvolatile semiconductor memory devicesthat can reduce the write voltage, has high capacity and realize a highspeed operation.

Furthermore, in the prior art, a selection gate of a selectiontransistor for selecting a memory cell is constituted by electricallyconnecting a control gate to a floating gate positioned in an endportion.

When a memory cell transistor and the selection transistor are formed ina two-layer gate structure, for the micronizing of the memory cell, aheight of a gate electrode needs to be increased in order to maintain acoupling capacitance equal to that before micronization. Therefore, inthis structure, the height of the gate electrode of the selectiontransistor also increases, thus it becomes difficult to process the gateelectrode.

Moreover, since the selection transistor requires superior cut-offcharacteristics as compared with the memory cell transistor, theselection transistor is formed in such a manner that a channel length issufficiently large as compared with the memory cell transistor.Thereafter, by the forming of the selection transistor, periodicity ofthe memory cell largely collapses, and it becomes difficult to adjustthe channel lengths of the memory cells positioned on opposite ends of amemory cell array. Furthermore, a problem occurs that a margin oflithography of the memory cell array drops.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided anonvolatile semiconductor memory comprising: a memory cell including afloating gate constituted of a first conductive layer formed on asemiconductor substrate via a gate insulation film, a pair of firstdiffusion layers which are source or drain regions formed in thesubstrate positioned on opposite sides of the floating gate, first andsecond control gates constituted of second conductive layers formed onthe opposite sides of the floating gate to drive the floating gate, andan inter-gate insulation film formed between the first and secondcontrol gates and the floating gate; and a selection transistor forselecting the memory cell, including a selection gate•wiring including afirst portion constituted of the same conductive layer as the firstconductive layer, disposed adjacent to one of the first and secondcontrol gates via the inter-gate insulation film, and formed on thesubstrate via the gate insulation film, and a second portion constitutedof the same conductive layer as the second conductive layer, disposedadjacent to the first portion, electrically connected to the firstportion, and formed on the substrate via an insulation film, and asecond diffusion layer formed in the substrate, facing the secondportion of the selection gate•wiring.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a plan view showing an example of a conventional nonvolatilesemiconductor memory device;

FIG. 2 is a cross-sectional view of FIG. 1;

FIG. 3 is another cross-sectional view of FIG. 1;

FIG. 4 is an equivalent circuit diagram of FIG. 1;

FIG. 5 is a plan view showing a constitution of a cell array in thenonvolatile semiconductor memory device according to a first embodiment;

FIG. 6 is a cross-sectional view of FIG. 5;

FIG. 7 is another cross-sectional view of FIG. 5;

FIG. 8 is a still another cross-sectional view of FIG. 5;

FIG. 9 is an equivalent circuit diagram of a cell of the firstembodiment;

FIGS. 10A to 10H are cross-sectional views successively showingmanufacturing steps of the nonvolatile semiconductor memory deviceaccording to the first embodiment;

FIG. 11 is a cross-sectional view showing a constitution of a cell arrayin the nonvolatile semiconductor memory device according to a secondembodiment;

FIG. 12 is a cross-sectional view showing a constitution of the cellarray in the nonvolatile semiconductor memory device according to athird embodiment;

FIGS. 13A to 13H are cross-sectional views successively showing themanufacturing steps of the nonvolatile semiconductor memory deviceaccording to the third embodiment;

FIG. 14 is a cross-sectional view showing a constitution of the cellarray in the nonvolatile semiconductor memory device according to afourth embodiment;

FIG. 15 is a cross-sectional view showing a constitution of the cellarray in the nonvolatile semiconductor memory device according to afifth embodiment;

FIGS. 16A to 16G are cross-sectional views successively showing themanufacturing steps of the nonvolatile semiconductor memory deviceaccording to the fifth embodiment;

FIG. 17 is a circuit diagram showing a general NAND type EEPROM;

FIG. 18 is a diagram showing an example of a potential in a case wheredata is written in the memory cell of FIG. 17;

FIG. 19 is a circuit diagram showing a constitution of the nonvolatilesemiconductor memory device according to the first to fifth embodiments;

FIG. 20 is a diagram showing an example of a potential applied to eachsection in a case where the data is written together with a schematicsection of the circuit of FIG. 19;

FIG. 21 is an equivalent circuit diagram showing an example of potentialsetting in a case where the data is written into the cell shown in FIG.19;

FIG. 22 is an equivalent circuit diagram showing another example of thepotential setting in a case where the data is written into the cellshown in FIG. 19;

FIG. 23 is a diagram showing an example of the writing of the data usingthe potential setting shown in FIG. 22;

FIG. 24 is a diagram showing an example of a potential applied to eachpart in a case where the data is erased in the nonvolatile semiconductormemory device according to the first to fifth embodiments;

FIG. 25 is a diagram showing an example of a potential applied to eachpart in a case where the data is read in the nonvolatile semiconductormemory device according to the first to fifth embodiments;

FIG. 26 is a diagram showing another example of a potential applied toeach part in a case where the data is read in the nonvolatilesemiconductor memory device according to the first to fifth embodiments;and

FIG. 27 is a block diagram showing a constitution of an applicationexample of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described herein-after in accordance withembodiments.

(First Embodiment)

FIGS. 5 to 8 show a constitution of a cell array in a nonvolatilesemiconductor memory device according to a first embodiment. FIG. 5 is aplan view of the cell array, and FIGS. 6 to 8 are cross-sectional viewsshowing different sections of FIG. 5.

A cell array according to a first embodiment comprises a plurality ofmemory cells connected in series to constitute a memory cell row, and aselection transistor for selecting the memory cell, connected to thememory cell row.

A memory cell comprises: a floating gate 13 (FG) formed on a siliconsemiconductor substrate 11 via a gate insulation film 12; a pair ofdiffusion layers 14 which are source or drain regions positioned onopposite sides of the floating gate 13 and formed in the substrate 11;and first and second control gates (CG) 16 formed on the opposite sidesof the floating gate 13 via inter-gate insulation films 15 are extended.It is to be noted that the first and second control gates (CG) 16 areshared between adjacent memory cells. The inter-gate insulation films 15are formed to surround peripheries of the first and second control gates16 except upper portions of the gates, that is, to extend to positionsbetween the control gates and the diffusion layers 14. Furthermore,metal salicide layers 17 are formed on the first and second controlgates 16. Mask layers 18 formed of insulation materials are formed onthe floating gates 13.

A selection gate•wiring 23 are formed in the selection transistor forselecting the memory cell. The selection gate•wiring 23 comprise a firstportion 19, a second portion 21, and a third portion 22 which areelectrically connected to one another. The first portion 19 comprisesthe same conductive layer as that of the floating gate 13 and isdisposed adjacent to one of the first and second control gates 16 viathe inter-gate insulation film 15 and is formed on the substrate 11 viathe gate insulation film 12. The second portion 21 comprises the sameconductive layer as that of the control gate 16 and is disposed adjacentto the first portion 19 and is electrically connected to the firstportion 19 and is formed on the substrate 11 on an insulation film 20.The third portion 22 comprises the same conductive layer as that of thecontrol gate 16 and is disposed adjacent to the second portion 21 and iselectrically connected to the second portion 21 and is formed on thesubstrate 11 via the gate insulation film 12.

Moreover, the mask layers 18 are formed on the first and third portions19, 22 of the selection gate•wiring 23 in the same manner as in thefloating gate 13, and the metal salicide layer 17 is formed on thesecond portion 21 of the selection gate•wiring 23. The diffusion layer14 is formed in the substrate 11 facing the second portion 21. Adiffusion layer 24 is formed in the silicon semiconductor substrate 11on one of the opposite sides of the third portion 22, opposite to a sideon which the diffusion layer 14 is formed. The diffusion layer 24 isconnected to a common source line or bit line. The metal salicide layer17 is also formed on the surface of the diffusion layer 24.

A spacer 25 comprising the insulation material is formed on a side wallof the third portion 22 of the selection gate•wiring 23. The spacer 25may comprise a single layer of insulation film, or a plurality of layersof insulation films. Furthermore, an interlayer insulation film 26 isdeposited on the whole surface of the substrate.

As shown in FIGS. 7 and 8, a shallow trench 27 for shallow trenchisolation (STI) extended in parallel with an arrangement direction ofthe diffusion layers 14, 24 is formed in the substrate 11, and aninsulation film 28 for isolating devices is buried in the trench 27 toisolate a plurality of memory cell rows from one another.

A floating gate has heretofore been driven by a control gate in theconventional cell. On the other hand, in the memory cell of the firstembodiment, the floating gate 13 is driven by two control gates 16, 16positioned on the opposite sides of the floating gate.

Moreover, the selection transistor comprises the selection gate•wiring23, the diffusion layer 14 positioned under the second portion 21, thediffusion layer 24, a channel region between a pair of diffusion layers14, and a channel region between the diffusion layers 14, 24. Here, thefirst portion 19 and the third portion 22 of the selection gate•wiring23 work as the selection gate, and the second portion 21 is used as awiring for connecting a plurality of selection gates to one another.That is, a driving voltage is applied to the second portion 21, thevoltages are supplied to the first portion 19 and the third portion 22positioned on the opposite sides of the second portion, accordingly aninversion channel is formed on the surface of the substrate 11positioned under the first portion 19 and third portion 22, thediffusion layer 14 in a memory cell row end portion is electricallyconnected to the diffusion layer 24, and the memory cell row isselected.

FIG. 9 shows an equivalent circuit of the cell of the first embodiment.Here, Cip represents a capacitance between two control gates CG and afloating gate FG, Cip_ext represents a capacitance between two controlgates CG and a substrate, and Ctox represents a capacitance between thefloating gate FG and the substrate. In this equivalent circuit, assumingthat two control gates CG adjacent to the floating gate FG have an equalpotential (Vcg), a capacitance ratio (Cr) which determines a potentialVfg of the floating gate is roughly estimated by the following equation:$\begin{matrix}{{Cr} = {{Cip}/\left( {{Cip} + {Ctox}} \right)}} \\{= {\left( {2 \times ɛ\;{ip} \times W \times {{Tfg}/{Tip}}} \right)/\left( {\left( {2 \times ɛ\;{ip} \times W \times {{Tfg}/{Tip}}} \right) +} \right.}} \\{\left. {ɛ\;{tox} \times W \times {L/{Ttox}}} \right),}\end{matrix}$where sip represents a permittivity of an inter-gate insulation film,εtox represents a permittivity of a gate insulation film, W represents achannel width of a cell transistor, L represents a gate length of thecell transistor, Tfg represents a film thickness of the floating gateFG, Ttox represents a film thickness of the gate insulation film, andTip represents a film thickness of the inter-gate insulation film.

It is seen from the above equation that Cr can be increased byincreasing the film thickness Tfg of the floating gate 13 withoutchanging the channel width or the gate length of the transistor, whichshould be a minimum design dimension in the cell transistor of thepresent embodiment. This means that the capacitance ratio can beimproved, even when the cell is micronized.

Moreover, as shown in FIG. 6, a space between the floating gates 13 issubstantially completely filled with the control gate 16. Therefore, acoupling capacitance between the floating gates adjacent to each otherin a word line WL direction, which has heretofore been a problem in theconventional cell, and two parasitic capacitances of a fringecapacitance between the substrate and the floating gate aresubstantially shielded.

From the above, in the cell of the first embodiment, the capacitanceratio can be secured by increasing the film thickness of the floatinggate 13 without considering any increase of the parasitic capacitance.As a result, even when the gate length or the channel width of the celltransistor is micronized, the capacitance ratio can be increased.Additionally, since the capacitance ratio can be increased, a writevoltage can be reduced. Therefore, according to the first embodiment,the micronizing of the cell and the reducing of the write voltage can besimultaneously satisfied.

Furthermore, since the selection gate•wiring 23 of the selectiontransistor do not have a conventional two-layer structure, and have asingle-layer structure, the height of the gate electrode decreases, andthe gate electrode can be easily processed.

Additionally, in the selection gate•wiring 23 of the selectiontransistor, the second portion 21 having substantially the samestructure as that of the control gate 16 of the memory cell row in themiddle, and the first and third portions 19, 22 having substantially thesame structure as that of the floating gate 13 of the memory cell roware arranged in one row, and are connected to one another. That is, theselection transistor has a sufficiently large channel length as comparedwith the memory cell transistor, and therefore has superior cut-offcharacteristics as compared with the memory cell transistor.

Next, a method of manufacturing the nonvolatile semiconductor memorydevice according to the first embodiment will be described withreference to FIGS. 10A to 10H. It is to be noted that FIGS. 10A to 10Acorrespond to a cross-sectional view of FIG. 6.

First, as shown in FIG. 10A, the gate insulation film 12 is formed, forexample, of any one of silicon oxide, silicon nitride, aluminum oxide,hafnium oxide, and zirconia oxide, or of a stacked film of at least twoof them on the silicon semiconductor substrate 11. Conductive materialsconstituting the floating gates, such as a polysilicon layer 30 and amask layer 18 are successively formed on the gate insulation film 12.For example, insulation materials such as a silicon oxide film and asilicon nitride film are used in the mask layer 18. The mask layer 18has a minimum condition that a selectivity to the polysilicon layer 30is obtained in etching the polysilicon layer 30. However, it is morepreferable that a selectivity to the insulation film for the burying beobtained in a chemical mechanical polishing (CMP) step during theforming of STI described later and that a selectivity to the controlgate be obtained in the CMP step during the forming of the control gate.Thereafter, the mask layer 18 is patterned by a lithography step and aselective etching step. The polysilicon layer 30, gate insulation film12, and substrate 11 are successively etched using the patterned masklayer 18 to form the shallow trench 27 for isolating the devices asshown in FIGS. 7 and 8. Next, the insulation film 28 formed, forexample, of a silicon oxide film is formed on the whole surface, forexample, by chemical vapor deposition (CVD) to fill the trench 27 (shownin FIGS. 7 and 8) formed in the substrate 11. Subsequently, theinsulation film 28 is polished down to the mask layer 18 by the CMP stepusing the mask layer 18 as a stopper, and the STI is formed.

Subsequently, as shown in FIG. 10B, the lithography step and selectiveetching step are performed to etch the mask layer 18, polysilicon layer30, and gate insulation film 12, and the floating gate is formed. As aresult, in the memory cell array, the floating gate 13 of each memorycell is formed of the polysilicon layer 30. At this time, the firstportion 19 of the selection gate•wiring 23 is formed by the polysiliconlayer 30 in such a manner that the first portion is disposed adjacent tothe floating gate 13 in an endmost portion of the memory cell row.Between the pair of first portions 19, the mask layer 18, polysiliconlayer 30, and gate insulation film 12 are left for a space for formingthe third portion 22 of the selection gate•wiring 23 and the anddiffusion layer 24 shown in FIG. 6. Subsequently, an oxide film isformed on the whole surface, impurity ions are injected into thesubstrate 11, and the diffusion layers 14 are formed to constitutesource/drain regions (S/D) of the cell transistors. Next, after theoxide film used in injecting the ions is removed, the inter-gateinsulation film 15 is formed on the whole surface. The inter-gateinsulation film 15 is formed, for example, by any one of silicon oxide,silicon nitride, aluminum oxide, hafnium oxide, and zirconia oxide, or astacked film of at least two of them. As an example, a so-called ONOfilm of three layers including silicon oxide, silicon nitride, andsilicon oxide may be used.

Subsequently, as shown in FIG. 10C, a mask layer 31 is formed having apattern for continuously covering the memory cell row and a part of thefirst portion 19 of the selection gate•wiring 23, and the inter-gateinsulation film 15 is removed in a masked state with the mask layer 31.

Thereafter, as shown in FIG. 10D, after removing the mask layer 31 andfurther forming the insulation film 20 on the exposed surface of thesubstrate, a conductive material forming the control gate and a part ofthe selection gate•wiring 23, such as a polysilicon layer 32, is formedby the CVD step.

Next, as shown in FIG. 10E, the polysilicon layer 32 is polished down tothe mask layer 18 on the floating gate 13 by the CMP step, and flatted.At this time, the inter-gate insulation film 15 left on the mask layer18 is also removed. Accordingly, in the memory cell row, the controlgate 16 is formed of the polysilicon layer 32 between the floating gates13, and the second portion 21 formed of the polysilicon layer 32 isdisposed adjacent to the first portion 19 of the selection gate•wiring23 in a region of the selection transistor.

Next, as shown in FIG. 10F, a resist layer is formed on the wholesurface, the lithography step and the selective etching step areperformed, and the mask layer 18, polysilicon layer 30 and lower gateinsulation film 12 left in the region of the selection transistor areetched using the patterned resist layer as a mask. As a result, thethird portion 22 is formed of the polysilicon layer 30 and disposedadjacent to the insulation film 20 of the selection gate•wiring 23 inthe region of the selection transistor. Subsequently, after removing theresist layer from the whole surface, the insulation material for forminga spacer is deposited on the whole surface, thereafter the insulationmaterial is etched by RIE, and the spacer 25 is formed on the side wallof the third portion 22. As described above, the spacer 25 may comprisea single insulation film, or a plurality of layers of insulation films.After forming the spacer 25 and forming the oxide film on the wholesurface, the impurity ions are injected into the substrate 11 to formthe diffusion layer 24 which is the source/drain (S/D) region of theselection transistor. Thereafter, the oxide film used in injecting theions is removed.

Next, as shown in FIG. 10G, a metal thin film is deposited on the wholesurface, and thereafter heated, and the mask layer 18 is used as acontrol film for a salicide reaction. Accordingly, the metal salicidelayer 17 is formed on the control gate 16, and the metal salicide layers17 are formed on the second portion 21 of the selection gate•wiring 23and on the surface of the diffusion layer 24. Non-reacted metal thinfilms are thereafter removed.

Thereafter, as shown in FIG. 10H, the interlayer insulation film 26 isformed on the whole surface, an opening connected to the surface of thediffusion layer 24 is further formed in the interlayer insulation film26, and a contact plug is formed to fill in the opening.

It is to be noted that a case where the control gate 16 has a salicidestructure has been described, but in the present embodiment, the controlgate 16 is formed in a self-aligned manner with respect to the floatinggate 13, and therefore a metal wiring may also be formed on the uppersurface of the control gate 16. Furthermore, the control gate 16 itselfmay also be formed of a metal material. In this embodiment, thefollowing materials are applicable to the control gate 16 and the secondportion 21 of the selection gate•wiring 23.

Examples of a metal material applied to the salicide structure includetitanium, cobalt, and nickel. When the control gate 16 and the secondportion 21 of the selection gate•wiring 23 are formed of metalmaterials, for example, any one of titanium, tungsten, tungsten nitride,and titanium nitride or a stacked film of at least two of them isapplicable.

In the present embodiment, the control gates 16 are formed on theopposite side surfaces of the floating gate 13 via the inter-gateinsulation films 15. Therefore, capacitive coupling of the floating gate13 and control gates 16 increases as compared with the prior art. Thewiring of the control gate 16 needs to be a material having asufficiently low resistance value, and the metal salicide layer 17formed on the control gate is useful for lowering the wiring resistancevalue of the control gate 16.

Moreover, the selection gate•wiring 23 of the selection transistor areformed following an arranged state of the floating gate 13 and controlgate 16 of the memory cell row as such. Therefore, when the selectiontransistor is formed, there is no possibility that periodicity of thememory cell collapses or that the channel lengths of the memory cellspositioned on opposite ends of the memory cell row fluctuate.Accordingly, a problem that a margin of lithography of the memory cellrow drops is solved.

(Second Embodiment)

FIG. 11 shows a constitution of a cell array in a nonvolatilesemiconductor memory device according to a second embodiment, andcorresponds to a cross-sectional view of FIG. 6 in the first embodiment.It is to be noted that in FIG. 11, portions corresponding to those ofFIG. 6 are denoted with the same reference numerals, and the descriptionis omitted.

In the cell array according to the first embodiment, a case where theselection gate•wiring 23 comprise three portions including the first,second, and third portions 19, 21, 22 has been described. On the otherhand, in the second embodiment, the selection gate•wiring 23 comprisesthe first and second portions 19, 21, and the third portion 22 isomitted. In this constitution, the first portion 19 serves as theselection gate, and the second portion 21 serves as a wiring forsupplying a driving voltage to the selection gate.

It is to be noted that in the second embodiment, the diffusion layer 14formed in the substrate 11 under the second portion 21 is connected tothe diffusion layer 24, and the spacer 25 is formed on the side wall ofthe second portion 21.

Even in the memory cell in the second embodiment, the floating gate 13is driven by two control gates 16, 16 positioned on the opposite sidesof the floating gate 13. Therefore, in the cell of the secondembodiment, when the film thickness of the floating gate is increasedwithout considering any increase of a parasitic capacitance, acapacitance ratio can be secured. As a result, even when the gate lengthor the channel width of the cell transistor is micronized, thecapacitance ratio can be increased. Additionally, since the capacitanceratio can be increased, a write voltage can be reduced. Therefore,according to the second embodiment, the micronizing of the cell and thereducing of the write voltage can be simultaneously satisfied.

Furthermore, since the selection gate•wiring 23 of the selectiontransistor do not have a two-layer structure as in the prior art, andhave a single-layer structure, the height of the gate electrodedecreases, and the gate electrode can be easily processed.

Additionally, in the selection gate•wiring 23 of the selectiontransistor, the first portion 19 having substantially the same structureas that of the floating gate 13 of the memory cell row, and the secondportion 21 having substantially the same structure as that of thecontrol gate 16 are arranged in a row, and are connected to each other.That is, the selection transistor has a sufficiently large channellength as compared with the memory cell transistor, and therefore hassuperior cut-off characteristics as compared with the memory celltransistor.

The nonvolatile semiconductor memory device according to the secondembodiment can be manufactured, when all the polysilicon layers 30contacting the second portions 21 and the mask layers 18 on thepolysilicon layers are removed in the step of FIG. 10F during themanufacturing of the nonvolatile semiconductor memory device accordingto the first embodiment, and thereafter steps similar to those of thefirst embodiment are performed.

(Third Embodiment)

FIG. 12 shows a constitution of a cell array in a nonvolatilesemiconductor memory device according to a third embodiment, andcorresponds to a cross-sectional view of FIG. 6 in the first embodiment.It is to be noted that in FIG. 12, portions corresponding to those ofFIG. 6 are denoted with the same reference numerals, and the descriptionis omitted.

In the cell array according to the first embodiment, direct contact ofthe first, second, and third portions 19, 21, 22 constituting theselection gate•wiring 23 have been described. On the other hand, in thethird embodiment, the inter-gate insulation film 15 is left in theperiphery of the second portion 21 to surround the second portion, andthe second portion 21 does not directly contact the first and thirdportions 19, 22.

Then, in the third embodiment, a part of the upper portion of theinter-gate insulation film 15 formed around the second portion 21 isremoved together with a part of the mask layer 18 which contacts theinter-gate insulation film. In the surface accordingly exposed, to formthe metal salicide layer 17 on the second portion 21, the metal salicidelayer 17 is formed to simultaneously connect the first, second, andthird portions 19, 21, 22 to one another via the metal salicide layer17.

That is, the selection gate•wiring 23 include the first portion 19, thesecond portion 21, the third portion 22, and the metal salicide layer 17which is a conductive film for electrically connecting these first,second, and third portions 19, 21, 22 to one another. The first portion19 comprises the same conductive layer as that of the control gate 16and is disposed adjacent to the control gate 16 and is formed on thesubstrate 11 via the gate insulation film 12. The second portion 21comprises the same conductive layer as that of the control gate 16, andcontacts the first portion 19 via the inter-gate insulation film 15. Thethird portion 22 comprises the same conductive layer as that of thecontrol gate 16 and contacts the second portion 21 via the inter-gateinsulation film 15 and is formed on the substrate 11 via the gateinsulation film 12. The metal salicide layer 17 is formed tocontinuously coat a part of the upper portion of the first portion 19,upper side surfaces and upper portion of the second portion 21, and apart of the upper portion of the third portion 22.

Even in the memory cell in the third embodiment, the floating gate 13 isdriven by two control gates 16, 16 positioned on the opposite sides ofthe floating gate 13. Therefore, in the cell of the third embodiment,when the film thickness of the floating gate is increased withoutconsidering any increase of the parasitic capacitance, a capacitanceratio can be secured. As a result, even when the gate length or thechannel width of the cell transistor is micronized, the capacitanceratio can be increased. Additionally, since the capacitance ratio can beincreased, a write voltage can be reduced. Therefore, according to thethird embodiment, the micronizing of the cell and the reducing of thewrite voltage can be simultaneously satisfied.

Furthermore, since the selection gate•wiring 23 of the selectiontransistor do not have a two-layer structure as in the prior art, andhave a single-layer structure, the height of the gate electrodedecreases, and the gate electrode can be easily processed.

Additionally, in the selection gate•wiring 23 of the selectiontransistor, the second portion 21 having substantially the samestructure as that of the control gate 16 in the middle, and the first,third portions 19, 22 having substantially the same structure as that ofthe floating gate 13 of the memory cell row are arranged in a row, andare connected to one another. That is, the selection transistor has asufficiently large channel length as compared with the memory celltransistor, and therefore has superior cut-off characteristics ascompared with the memory cell transistor.

Next, a method of manufacturing the nonvolatile semiconductor memorydevice according to the third embodiment will be described withreference to FIGS. 13A to 13H. It is to be noted that FIGS. 13A to 13Acorrespond to a cross-sectional view of FIG. 6.

First, as shown in FIG. 13A, the gate insulation film 12 is formed, forexample, of silicon oxide on the silicon semiconductor substrate 11. Forexample, a polysilicon layer 30 and a mask layer 18 are successivelyformed on the gate insulation film 12. For example, insulation materialssuch as a silicon oxide film and a silicon nitride film are used in themask layer 18. Thereafter, the mask layer 18 is patterned by alithography step and a selective etching step. The polysilicon layer 30,gate insulation film 12, and substrate 11 are successively etched usingthe patterned mask layer 18 to form the shallow trench 27 for isolatingthe devices as shown in FIGS. 7 and 8. Next, the insulation film 28(shown in FIGS. 7 and 8) formed, for example, of a silicon oxide film isformed on the whole surface, for example, by CVD to fill the trench 27formed in the substrate 11. Subsequently, the insulation film 28 ispolished down to the mask layer 18 by the CMP step using the mask layer18 as a stopper, and the STI is formed.

Subsequently, as shown in FIG. 13B, the lithography step and selectiveetching step are performed to etch the mask layer 18, polysilicon layer30, and gate insulation film 12, and the floating gate is formed. As aresult, in the memory cell row, the floating gate 13 of each memory cellis formed of the polysilicon layer 30. At this time, the first portion19 of the selection gate•wiring 23 is formed by the polysilicon layer 30in such a manner that the portion is disposed adjacent to the floatinggate 13 in an endmost portion of the memory cell row. Between the pairof first portions 19, the mask layer 18, polysilicon layer 30, and gateinsulation film 12 are left for a space for forming the third portion 22of the selection gate•wiring 23 and the diffusion layer 24 shown in FIG.12. Subsequently, an oxide film is formed on the whole surface, impurityions are injected into the substrate 11, and the diffusion layers 14 areformed to constitute source/drain regions (S/D) of the cell transistors.Next, after the oxide film used in injecting the ions is removed, theinter-gate insulation film 15 is formed on the whole surface. Theinter-gate insulation film 15 is formed, for example, by any one ofsilicon oxide, silicon nitride, aluminum oxide, hafnium oxide, andzirconia oxide, or a stacked film of at least two of them. As anexample, a so-called ONO film of three layers including silicon oxide,silicon nitride, and silicon oxide may be used.

Subsequently, as shown in FIG. 13C, for example, a polysilicon layer 32constituting the control gate and a part of the selection gate•wiring isformed on the whole surface by CVD.

Subsequently, as shown in FIG. 13D, the polysilicon layer 32 is polisheddown to the mask layer 18 on the floating gate 13 by the CMP step, andflatted. Subsequently, the inter-gate insulation film 15 left on themask layer 18 is removed. Accordingly, in the memory cell row, thecontrol gate 16 is formed of the polysilicon layer 32 between thefloating gates 13, and the second portion 21 formed of the polysiliconlayer 32 is formed and disposed adjacent to the first portion 19 of theselection gate•wiring 23 in a region of the selection transistor.

Next, as shown in FIG. 13E, a resist layer is formed on the wholesurface, the lithography step and the selective etching step areperformed, and the mask layer 18, polysilicon layer 30 and lower gateinsulation film 12 left in the region of the selection transistor areetched using the patterned resist layer as a mask. As a result, thethird portion 22 is formed of the polysilicon layer 30 and disposedadjacent to the insulation film 20 of the selection gate•wiring 23 inthe region of the selection transistor. Subsequently, after removing theresist layer, the insulation material for forming a spacer is depositedon the whole surface, thereafter the insulation material is etched byRIE, and the spacer 25 is formed on the side wall of the third portion22. After forming the spacer 25 and forming the oxide film on the wholesurface, the impurity ions are injected into the substrate 11 to formthe diffusion layer 24 which is the source/drain (S/D) region of theselection transistor. Thereafter, the oxide film used in injecting theions is removed.

Next, as shown in FIG. 13F, a mask layer is formed, and the etching isperformed using the mask layer. Accordingly, a part of the upper portionof the inter-gate insulation film 15 formed around the second portion 21of the selection gate•wiring 23 is removed together with a part of themask layer 18 which contacts the inter-gate insulation film, andopenings 33 are formed.

Next, as shown in FIG. 13G, a metal thin film is deposited on the wholesurface, and thereafter heated, and the mask layer 18 is used as acontrol film for a salicide reaction. Accordingly, the metal salicidelayer 17 is formed on the control gate 16, and the metal salicide layers17 are formed on the second portion 21 of the selection gate•wiring 23and on the surface of the diffusion layer 24. At this time, the metalsalicide layer 17 is formed to simultaneously coat a part of the upperportion of the first portion 19, upper side surfaces and upper portionof the second portion 21, and a part of the upper portion of the thirdportion 22 continuously. Non-reacted metal thin films are thereafterremoved.

Next, as shown in FIG. 13H, the interlayer insulation film 26 is formedon the whole surface, an opening connected to the surface of thediffusion layer 24 is further formed in the interlayer insulation film26, and a contact plug is formed to fill in the opening.

In the present embodiment, the control gates 16 are formed on theopposite side surfaces of the floating gate 13 via the inter-gateinsulation films 15. Therefore, capacitive coupling of the floating gate13 and control gates 16 increases as compared with the prior art. Thewiring of the control gate 16 needs to be a material having asufficiently low resistance value, and the metal salicide layer 17formed on the control gate is useful for lowering the wiring resistancevalue of the control gate 16.

Moreover, the selection gate•wiring 23 of the selection transistor areformed following an arranged state of the floating gate 13 and controlgate 16 of the memory cell row as such. Therefore, when the selectiontransistor is formed, there is no possibility that periodicity of thememory cell collapses or that the channel lengths of the memory cellspositioned on opposite ends of the memory cell row fluctuate.Accordingly, a problem that a margin of lithography of the memory cellrow drops is solved.

It is to be noted that after the step of FIG. 13G, a metal for wiring isformed on the whole surface, and patterned to form the wiring on themetal salicide layer 17 of the selection gate•wiring 23, so that wiringresistance can be reduced.

(Fourth Embodiment)

FIG. 14 shows a constitution of a cell array in a nonvolatilesemiconductor memory device according to a fourth embodiment, andcorresponds to a cross-sectional view of FIG. 12 in the thirdembodiment. It is to be noted that in FIG. 14, portions corresponding tothose of FIG. 12 are denoted with the same reference numerals, and thedescription is omitted.

In the cell array according to the third embodiment, a case where theselection gate•wiring 23 comprise the first, second, and third portions19, 21, 22 and the metal salicide layer 17 for connecting the portionsto one another has been described. On the other hand, in the fourthembodiment, the selection gate•wiring 23 comprise the first and secondportions 19, 21 and the metal salicide layer 17 for connecting theportions to each other, and the third portion 22 is omitted.

In this constitution, the first portion 19 serves as the selection gate,and the second portion 21 serves as a wiring for supplying a drivingvoltage to the selection gate.

It is to be noted that in the fourth embodiment, the diffusion layer 14formed in the substrate 11 under the second portion 21 is connected tothe diffusion layer 24, and the spacer 25 is formed on the side wall ofthe second portion 21. In the present embodiment, a case where wirings34 are formed on the metal salicide layers 17 in the first and secondportions 19, 21 of the selection gate•wiring 23 will be described.

Even in the memory cell in the fourth embodiment, the floating gate 13is driven by two control gates 16, 16 positioned on the opposite sidesof the floating gate 13. Therefore, in the cell of the fourthembodiment, when the film thickness of the floating gate is increasedwithout considering any increase of a parasitic capacitance, acapacitance ratio can be secured. As a result, even when the gate lengthor the channel width of the cell transistor is micronized, thecapacitance ratio can be increased. Additionally, since the capacitanceratio can be increased, a write voltage can be reduced. Therefore,according to the fourth embodiment, the micronizing of the cell and thereducing of the write voltage can be simultaneously satisfied.

Furthermore, since the selection gate•wiring 23 of the selectiontransistor do not have a two-layer structure as in the prior art, andhave a single-layer structure, the height of the gate electrodedecreases, and the gate electrode can be easily processed.

Additionally, in the selection gate•wiring 23 of the selectiontransistor, the first portion 19 having substantially the same structureas that of the floating gate 13 of the memory cell row, and the secondportion 21 having substantially the same structure as that of thecontrol gate 16 are arranged in a row, and are connected to each other.That is, the selection transistor has a sufficiently large channellength as compared with the memory cell transistor, and therefore hassuperior cut-off characteristics as compared with the memory celltransistor.

The nonvolatile semiconductor memory device according to the fourthembodiment can be manufactured, when all the polysilicon layers 30contacting the second portions 21 and the mask layers 18 on thepolysilicon layers are removed in the step of FIG. 13E during themanufacturing of the nonvolatile semiconductor memory device accordingto the third embodiment, and thereafter steps similar to those of thethird embodiment are performed.

(Fifth Embodiment)

FIG. 15 shows a constitution of a cell array in a nonvolatilesemiconductor memory device according to a fifth embodiment, andcorresponds to a cross-sectional view of FIG. 6 in the first embodiment.

A memory cell comprises: a floating gate 13 (FG) formed on a siliconsemiconductor substrate 11 via a gate insulation film 12; diffusionlayers 14 which are source or drain regions formed in the substrate 11positioned on opposite sides of the floating gate 13; and first andsecond control gates (CG) 16 formed on the opposite sides of thefloating gate 13 via an inter-gate insulation film 15. It is to be notedthat the first and second control gates (CG) 16 are shared betweenadjacent two memory cells. A sectional shape of each floating gate 13 isformed in such a manner that a lower part of the section is broader thanan upper part, and the inter-gate insulation film 15 is formed along theside surface of each floating gate 13.

The inter-gate insulation films 15 are formed to surround peripheries ofthe first and second control gates 16 except upper surfaces of thegates, that is, to extend to positions between the control gates and thediffusion layers 14. In this case, unlike the first embodiment, metalsalicide layers 17 are not formed on the first and second control gates16. Mask layers 18 formed of insulation materials are formed on thefloating gates 13.

The selection transistor includes a first portion 19 which is formed ofthe same conductive layer as that of the floating gate 13 and which isdisposed adjacent to one of the first and second control gates 16 viathe inter-gate insulation film 15 and which is formed on the substrate11 via the gate insulation film 12. The first portion 19 is formed insuch a manner that a lower part of a sectional shape is broader than anupper part in the same manner as in the floating gate 13, and theinter-gate insulation film 15 is formed along the side surface of thefirst portion 19 adjacent to the control gate 16. The selectiontransistor includes a second portion 21 which is formed of the sameconductive layer as that of the control gate 16 and which is disposedadjacent to the first portion 19 and which is electrically connected tothe first portion 19 in a part where no mask layer 18 is formed, andwhich is formed on the substrate 11 via the gate insulation film 12.Moreover, the first portion 19 and the second portion 21 constitute theselection gate•wiring 23 of the selection transistor.

A mask layer 18 is formed on the first portion 19 of the selectiongate•wiring 23 in the same manner as in the floating gate 13. In thiscase, unlike the first embodiment, the metal salicide layer 17 is notformed on the second portion 21 of the selection gate•wiring 23, but maybe formed if necessary. A diffusion layer 24 is extended and formed inthe substrate 11 facing the second portion 21. The diffusion layer 24 isintegrated with the selection transistor, connected to the adjacentmemory cell row. The diffusion layer 24 is connected to a common sourceline or a bit line. Furthermore, the interlayer insulation film 26 isdeposited on the whole surface of the substrate.

In the conventional cell, a floating gate has heretofore been driven bya control gate. On the other hand, even in the memory cell of the fifthembodiment, the floating gate 13 is driven by two control gates 16, 16positioned on the opposite sides of the floating gate.

Moreover, the selection transistor comprises the selection gate•wiring23, the diffusion layer 24 positioned under the second portion 21, thediffusion layer 14, and a channel region between the diffusion layers14, 24. Here, the first portion 19 of the selection gate•wiring 23serves as the selection gate, and the second portion 21 is used as awiring for connecting a plurality of selection gates to one another.That is, a driving voltage is applied to the second portion 21, thevoltage is also supplied to the first portion 19, accordingly ainversion channel is formed on the surface of the substrate 11positioned under the first portion 19, the diffusion layer 14 in amemory cell row end portion is electrically connected to the diffusionlayer 24, and the memory cell row is selected.

Moreover, as shown in FIG. 15, a space between the floating gates 13 issubstantially completely filled with the control gate 16. Therefore, acoupling capacitance between the floating gates adjacent to each otherin a word line WL direction, which has heretofore been a problem in theconventional cell, and two parasitic capacitances of a fringecapacitance between the substrate and the floating gate aresubstantially shielded.

From the above, in the cell of the fifth embodiment, the capacitanceratio can be secured, when the film thickness of the floating gate isincreased without considering any increase of the parasitic capacitance.As a result, even when the gate length or the channel width of the celltransistor is micronized, the capacitance ratio can be increased.Additionally, since the capacitance ratio can be increased, a writevoltage can be reduced. Therefore, even in the fifth embodiment, themicronizing of the cell and the reducing of the write voltage can besimultaneously satisfied.

Furthermore, since the selection gate•wiring 23 of the selectiontransistor do not have a two-layer structure as in the prior art, andhave a single-layer structure, the height of the gate electrodedecreases, and the gate electrode can be easily processed.

Next, a method of manufacturing the nonvolatile semiconductor memoryaccording to the fifth embodiment will be described with reference toFIGS. 16A to 16G.

First, as shown in FIG. 16A, the gate insulation film 12 is formed, forexample, of silicon oxide on the silicon semiconductor substrate 11.Conductive materials, such as a polysilicon layer 30 and a mask layer 18are successively formed on the gate insulation film 12. For example,insulation materials such as a silicon oxide film and a silicon nitridefilm are used in the mask layer 18. Thereafter, the mask layer 18 ispatterned by a lithography step and a selective etching step. Thepolysilicon layer 30, gate insulation film 12, and substrate 11 aresuccessively etched using the patterned mask layer 18 to form theshallow trench 27 for isolating the devices as shown in FIGS. 7 and 8.Next, the insulation film 28 formed, for example, of a silicon oxidefilm is formed on the whole surface to fill the trench 27 formed in thesubstrate 11. Subsequently, the insulation film 28 is polished down tothe mask layer 18 by the CMP step using the mask layer 18 as a stopper,and the STI is formed.

Subsequently, as shown in FIG. 16B, the lithography step and selectiveetching step are performed to etch the mask layer 18, and polysiliconlayer 30, and the floating gate is processed. In this case, when theetching of the polysilicon layer 30 is discontinued halfway, asdescribed above, the floating gate 13 and the first portion 19 of theselection gate•wiring 23 are formed in such a manner that the lower partof each sectional shape is broader than the upper part. Subsequently,impurity ions are injected into the substrate 11, the diffusion layers14 constituting source/drain (S/D) regions of the cell transistors areformed, and the diffusion layer 24 constituting the source/drain (S/D)region of the selection transistor is formed.

Next, as shown in FIG. 16C, the inter-gate insulation film 15 is formedon the whole surface. The inter-gate insulation film 15 is formed of,for example, one of silicon oxide, silicon nitride, aluminum oxide,hafnium oxide, and zirconia oxide, or a stacked film of at least two ofthem. As an example, a so-called ONO film of three layers includingsilicon oxide, silicon nitride, and silicon oxide may be used.

Subsequently, as shown in FIG. 16D, the inter-gate insulation films 15on the diffusion layer 24 and on a part of the first portion 19 of theselection gate•wiring 23 adjacent to the diffusion layer are removed.

Thereafter, as shown in FIG. 16E, a conductive material forming thecontrol gate and a part of the selection gate•wiring 23, such as apolysilicon layer 32, is formed by CVD.

Subsequently, as shown in FIG. 16F, the polysilicon layer 32 is polisheddown to the mask layer 18 on the floating gate 13 by a CMP step, andflatted. At this time, the inter-gate insulation film 15 left on themask layer 18 is also removed. Accordingly, in the memory cell row, thecontrol gate 16 is formed of the polysilicon layer 32 between thefloating gates 13. At this time, the polysilicon layer 32 is left on thediffusion layer 24.

Next, a resist layer is deposited on the whole surface, the lithographystep and the selective etching step are performed, and the polysiliconlayer 32 remaining in a region for the selection transistor is etchedusing the patterned resist layer as a mask as shown in FIG. 16G. As aresult, the second portion 21 is formed of the polysilicon layer 32 insuch a manner that the portion is disposed adjacent to the first portion19 of the selection gate•wiring 23 in the region of the selectiontransistor. Subsequently, the resist layer is removed, the interlayerinsulation film 26 is deposited on the whole surface as shown in FIG.15, an opening connected to the surface of the diffusion layer 24 isformed in the interlayer insulation film 26, and a contact plug isformed to fill in the opening.

Next, an operation of the nonvolatile semiconductor memory deviceaccording to the first to fifth embodiments will be described.

First, a general NAND type EEPROM will be described with reference toFIGS. 17, 18. FIG. 17 shows a circuit constitution of the NAND typeEEPROM, and FIG. 18 shows an example of a potential in a case where datais written in the memory cell in this NAND type EEPROM. In FIGS. 17 and18, the same components are denoted with the same reference numerals.

In the NAND type EEPROM, a plurality of memory rows are disposed. Ineach memory row, a plurality of cell transistors which are memory cellsMC are connected in series, a selection gate ST1 is connected betweenone end of the row and a bit line BL, and a selection gate ST2 isconnected between the other end of the row and a source line SRC.

A predetermined gate potential Vsg is applied to a selection gate lineSGD on the side of the bit line BL at a data write time. A sufficientlylow potential Vbl is supplied to the bit line BL. The gate potential Vsgis set to a potential at which the selection gate ST1 can besufficiently turned on with respect to Vbl. When Vbl is supplied to thebit line, the selection gate ST1 turns on, and Vbl is transmitted to thecell transistor. Therefore, a channel potential of the cell transistorsufficiently drops, and the data is written.

In the general EEPROM, at the data write time, capacitive couplingbetween the control gate and the floating gate is used in either anoperation for applying a write potential Vpgm to a selected word line WL(CG3 in FIG. 18) to write data into the cell or an operation forapplying a transfer potential Vpass to a non-selected word line WL(except CG3 in FIG. 18) to form a channel.

FIG. 19 shows a circuit constitution of the NAND type nonvolatilesemiconductor memory device in which a plurality of memory cells areconnected in series according to the first to fifth embodiments, andFIG. 20 shows an example of a potential applied to each part in a casewhere the data is written, together with a schematic section of thecircuit.

As described above, a floating gate FG shares two control gates CG, andthe floating gate FG is selected by two control gates CG. That is, thefloating gate FG is driven by the capacitive coupling with two controlgates CG.

At the data write time, the predetermined gate potential Vsg is appliedto the selection gate line SGD on the side of the bit line BL, and theselection gate ST1 is turned on. On the other hand, a low gatepotential, for example, 0 V is applied to a selection gate line SGS on asource line SRC side, and the selection gate ST2 is turned off.Furthermore, a sufficient low potential Vbl is supplied to the bit lineBL. The potential Vsg of the selection gate line SGD is set to such apotential that the selection gate ST1 can be sufficiently turned on withrespect to Vbl of the bit line BL. For example, the equal writepotential Vpgm is applied to two control gates CG positioned attached tothe floating gate FG of the write target cell in which the data iswritten, and the substrate (P-type substrate) is set, for example, to 0V. This equivalent circuit of the write target cell is shown in FIG. 21.In this state, charges are injected into the floating gate FG from thesubstrate.

As described in the first to fifth embodiments, the capacitance ratiocan be increased regardless of the micronizing of the device, and Vpgmcan be reduced as compared with the prior art.

It is to be noted that the potentials applied to each control gate CGand selection gates SGD, SGS are generated by a row decoder circuitwhich is a control gate driving circuit.

In the write operation, a case where the equal voltage is supplied totwo control gates CG to drive the floating gate FG has been described.However, different potentials may also be supplied to two control gatesCG.

FIG. 22 shows the equivalent circuit of the write target cell in a casewhere Vpgm is supplied to one control gate CG, and 0 V is supplied tothe other control gate CG. In FIG. 22, it is assumed that a capacitanceratio of Cip to Ctox is 1.5:1 and no charge is injected into thefloating gate FG in a neutral threshold voltage and the existingthreshold voltage is 0 V.

The potential Vfg of the floating gate FG shown in FIG. 21 is asfollows: $\begin{matrix}{{Vfg} = {{Vpgm} \times 2 \times {{Cip}/\left( {{2 \times {Cip}} + {Ctox}} \right)}}} \\{= {0.75 \times {Vpgm}}}\end{matrix}$

On the other hand, the potential Vfg of the floating gate FG shown inFIG. 22 is as follows: $\begin{matrix}{{Vfg} = {{Vpgm} \times {{Cip}/\left( {{2 \times {Cip}} + {Ctox}} \right)}}} \\{= {0.375 \times {Vpgm}}}\end{matrix}$

In this manner, when the potential of one of two control gates CG ischanged, the capacitance ratio can be largely controlled.

FIG. 23 shows an example of the writing of the data using theabove-described characteristics. In FIG. 23, Vpgm are applied to thecontrol gates CG on the opposite sides of the write target cell. By theuse of the above-described assumption, a potential of 0.75×Vpgm isapplied to the floating gate FG of the write target cell. While 0 V isapplied to one of two control gates CG positioned on the left side ofthe write target cell, and Vpgm is applied to the other control gate.Therefore, a potential of 0.375×Vpgm is applied to the floating gate FGof the cell positioned on the left side of the write target cell.Therefore, an electric field stress to the cell disposed on the leftside is ½ of that of the floating gate FG of the selected cell, which issufficient for suppressing any write error. A predetermined potentialVpass for boosting the channel potential is applied to the control gateCG2 further distant from the selected cell. At an actual deviceoperation time, the potentials of the control gates CG are appropriatelycombined in consideration of write characteristics, channel boostingcharacteristics, potential transfer characteristics and the like.

FIG. 24 shows a cross-sectional view of the nonvolatile semiconductormemory device according to the first to fifth embodiments, and shows anexample of the potential applied to each part in a case where the datais erased.

When the data is erased, the substrate (P-type substrate) on which thememory cells are formed is boosted to an erase potential Vera. Moreover,the diffusion layer connected to the bit line BL and source line SRC andthe selector gates SGS, SGD are boosted to the potential Vera equal tothat of the substrate in order to prevent collapse. Furthermore, asufficiently low potential, for example, 0 V is supplied to the controlgate CG positioned adjacent to the cell to be erased. Then, the chargesare pulled toward the boosted substrate from the floating gate FG, andthe data is erased.

It is to be noted that the control gate CG is floating state in anon-erased cell. In this case, the potential of the control gate CG isboosted to the substrate potential by the capacitive coupling with thesubstrate, and the data is inhibited from being erased.

In this manner, even in the memory having the cell structure in whichthe control gates CG are disposed on the opposite sides of the floatinggate FG, the data can be securely erased.

FIG. 25 shows a cross-sectional view of the nonvolatile semiconductormemory device according to the first to fifth embodiments, and shows anexample of the potential applied to each part in a case where the datais read.

In FIG. 25, read voltages Vwl are supplied to two control gates CG (CG4,CG5) adjacent to the floating gate FG (FG45) of a read target cell. Theread voltage Vwl is preferably set to an appropriate potential inconsideration of the write characteristics, data holdingcharacteristics, an operation range of a cell transistor thresholdvoltage and the like. Assuming that a read voltage Vwl=0 V is set, apotential of 0 V is applied to the floating gate FG of the read cell.

On the other hand, a potential Vread for passing a cell current isapplied to the control gate CG further adjacent to two control gates CGpositioned on the opposite sides of the read target cell. The potentialVread is preferably set to an appropriate potential in order to removean influence of the non-selected cell connected to the read target celland to judge the threshold voltage of the read target cell.

The bit line BL is connected to a sense amplifier circuit having a latchfunction, and the threshold voltage of the read target cell is judged inthe sense amplifier circuit to sense the data at the read time. Here, atthe read time, the threshold voltage is judged with respect to only thecell in which both the control gates CG disposed on the opposite sidesof the cell indicate a read voltage Vwl. On the other hand, the cell inwhich the potentials of two control gates CG indicate a combinationdifferent from the above-described combination is brought into anon-state regardless of the stored data.

FIG. 26 shows a cross-sectional view of the nonvolatile semiconductormemory device according to the first to fifth embodiments, and showsanother example of the potential applied to each part in a case wherethe data is read.

In this example, read voltages Vwl are supplied to two control gates CG(CG4, CG5) adjacent to the floating gate FG (FG45) of the read targetcell. A potential Vread2 for passing the cell current is applied to theother control gate CG (CG1, CG2, etc.).

Additionally, in each memory cell row of FIG. 19, a control gate CG9positioned in an endmost portion on a bit line BL side and adjacent tothe selector gate ST1 contacts the selector gate ST1 via the inter-gateinsulation film. Therefore, when a high voltage is applied to theselection gate ST1 at a write time or the like, there is a possibilitythat a withstand pressure defect occurs with the selection gate ST1.Then, a voltage lower than that of the other control gate is applied tothe control gate positioned in the endmost portion in the memory cellrow, for example, CG9 in FIG. 19. In this manner, a write speed into thefloating gate of the memory cell positioned in the endmost portion inthe memory cell row is retarded. To prevent this, the voltage applied tothe control gate positioned in the endmost portion on the bit line BLside in the memory cell row, that is, CG9 in FIG. 19 is set to be lowerthan a usual voltage only in a case where the data is written in thememory cell positioned in the endmost portion on the bit line BL side inthe memory cell row. A voltage to be applied to an inner control gate,that is, CG8 in FIG. 19 is set to be higher than the usual voltage. Thatis, by the use of a principle shown in FIG. 22, a withstand voltagebetween the control gate and the selection gate is prevented fromdropping, and the data can be written into the memory cell positioned inthe endmost portion on the bit line BL side in the memory cell row at aspeed equal to that at a time when the data is written into anothermemory cell.

On the other hand, in FIG. 19, at a write time of the data, a lowpotential, for example, 0 V is applied to the gate of the selection gateST2 connected to the source line SRC, and the selection gate ST2 isturned off. Here, the control gate CG1 positioned in the endmost portionon the source line SRC side and adjacent to the selection gate ST2 alsocontacts the selection gate ST2 via the inter-gate insulation film.Therefore, when a high voltage is applied to the control gate CG1 at thewrite time or the like, there is a possibility that the potential of theselection gate line SGS floats from 0 V by the capacitive couplingbetween the control gate and the selection gate ST2. Accordingly, at thedata write time, the selection gate ST2 which should be originallyturned off is not turned off, and a possibility occurs that the currentflows and sufficient write cannot be performed. Then, by the applicationof a voltage lower than that of the other control gate to the controlgate positioned in the endmost portion on the source line SRC side inthe memory cell row, for example, CG1 in FIG. 19, the potential of theselection gate line SGS is inhibited from rising. Additionally, in thiscase, the writing speed into the floating gate of the memory cellpositioned in the endmost portion on the source line SRC side in thememory cell row is retarded. To prevent this, only in a case where thedata is written into the memory cell positioned in the endmost portionon the source line SRC side in the memory cell row, the voltage to beapplied to the control gate positioned in the endmost portion on thesource line SRC side in the memory cell row, that is, CG1 in FIG. 19 isset to be lower than the usual voltage, and a voltage applied to theinner control gate, that is, CG2 in FIG. 19 is set to be higher than theusual voltage. That is, by the use of the principle shown in FIG. 22,the potential of the selection gate line SGS is prevented from rising,and data can be sufficiently written even in the memory cell positionedin the endmost portion on the source line SRC side in the memory cellrow.

Next, an application example of the nonvolatile semiconductor memorydevice of the present invention will be described. FIG. 27 is a blockdiagram showing a schematic constitution of a flash memory system towhich the nonvolatile semiconductor memory device of the presentinvention is applied. As shown, the flash memory system of theapplication example comprises a host platform 101 and a universal serialbus (USB) flash device 102.

The host platform 101 is connected to the USB flash device 102 via a USBcable 103. The host platform 101 is connected to the USB cable 103 via aUSB host connector 104, and the USB flash device 102 is connected to theUSB cable 103 via a USB flash device connector 105. The host platform101 includes a USB host control unit 106 which controls packettransmission on the USB bus 103.

The USB flash device 102 includes: a USB flash device control unit 107which controls other elements in the device 102 and which also controlsan interface of the device 102 into the USB bus 103; the USB flashdevice connector 105; and at least a flash memory module 108 includingthe nonvolatile semiconductor memory device of the present invention.

When the USB flash device 102 is connected to the host platform 101, astandard USB listing process starts. In the process, the host platform101 recognizes the USB flash device 102 to select a communication modewith the USB flash device 102, and transfers/receives data with respectto the USB flash device 102 via a FIFO buffer, which is referred to asan end point, and in which transfer data is stored. The host platform101 recognizes physical or electrical state changes such asdetachment/attachment of the USB flash device 102 via the other endpoint, and receives a packet to be received, if any.

When the host platform 101 sends a request packet to the USB hostcontrol unit 106 to request for services from the USB flash device 102.The USB host control unit 106 transmits the packet onto the USB cable103. When the USB flash device 102 has an end point that has receivedthe request packet, the requests are accepted by the USB flash devicecontrol unit 107.

The USB flash device control unit 107 performs various operations suchas read, write, and erase of the data with respect to the flash memorymodule 108. Moreover, basic USB functions such as acquisition of a USBaddress are supported. The USB flash device control unit 107 controlsthe flash memory module 108 via a control line 109 which controls anoutput of the flash memory module 108, or various signals such as a chipenable signal/CE and a read/write signal. The flash memory module 108 isconnected to the USB flash device control unit 107 via an address databus 110. The address data bus 110 transfers a command of read, write, orerase with respect to the flash memory module 108, and address and dataof the flash memory module 108.

To inform the host platform 101 of results and states with respect tovarious operations required by the host platform 101, the USB flashdevice 102 transmits a state packet using a state end point (end point0). In this process, the host platform 101 checks whether or not thereis a state packet (poling), and the USB flash device 102 returns a blankpacket, or the state packet itself, when there is no packet indicating anew state message.

Various functions of the USB flash device can be carried out asdescribed above. It is to be noted that in FIG. 27, the USB cable 103may be omitted to directly connect the connector 104 to the connector105.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventionconcept as defined by the appended claims and their equivalents. Forexample, a case where a plurality of memory cells are connected inseries, and connected to the NAND type has been described with referenceto FIG. 19, but a plurality of memory cell transistors may also beconnected to an AND type.

1. A nonvolatile semiconductor memory device comprising: a memory cellincluding a floating gate constituted of a first conductive layer formedon a semiconductor substrate via a gate insulation film, a pair of firstdiffusion layers which are source or drain regions formed in thesubstrate positioned on opposite sides of the floating gate, first andsecond control gates constituted of second conductive, layers formed onthe opposite aides of the floating gate to drive the floating gate, andan inter-gate insulation film formed between the first and secondcontrol gates and the floating gate; and a selection transistor forselecting the memory cell, including a selection gate•wiring including afirst portion constituted of the same conductive layer as the firstconductive layer, disposed adjacent to one of the first and secondcontrol gates via the inter-gate insulation film, and formed on thesubstrate via the gate insulation film, and a second portion constitutedof the same conducive layer as the second conductive layer, disposedadjacent to the first portion, electrically connected to the firstportion, and formed on the substrate via an insulation film, and asecond diffusion layer formed in the substrate, facing the secondportion of the selection gate•wiring.
 2. The nonvolatile semiconductormemory device according to clam 1, wherein the selection transistorfurther comprises: a third portion constituted of the same conductivelayer as the first conductive layer, disposed adjacent to the secondportion, electrically connected to the second portion, and formed on thesubstrate via the gate insulation film; and a third diffusion layerformed in the substrate on a side of the third portion opposite to aside on which the second diffusion layer is formed.
 3. The nonvolatilesemiconductor memory device according to claim 1, wherein the inter-gateinsulation film is extended to a position between each of the first andsecond control gates and the first diffusion layer.
 4. The nonvolatilesemiconductor memory device according to claim 1, further comprising: atrench which is formed in the substrate to extend in a directionparallel to an arrangement direction of the first and second diffusionlayers; and an insulation film which is termed in the trench.
 5. Thenonvolatile semiconductor memory device according to claim 1, whereinmetal salicide layers are formed on the first and second control gates.6. The nonvolatile semiconductor memory device according to claim 1,wherein each of the first and second conductive layers is formed ofpolysilicon.
 7. The nonvolatile semiconductor memory device according toclaim 1, wherein each of the first and second conductive layerscomprises one of titanium, tungsten, and titanium nitride, or a stackedfilm of at least two of them.
 8. The nonvolatile semiconductor memorydevice according to claim 1, wherein each of the first and secondconductive layers comprises a salicide structure of titanium, cobalt, ornickel metal.
 9. The nonvolatile semiconductor memory device accordingto claim 1, wherein the gate insulation film comprises one of siliconoxide, silicon nitride, aluminum oxide, hafnium oxide, and zirconiaoxide, or a stacked film of at least two of them.
 10. The nonvolatilesemiconductor, memory device according to claim 1, wherein theinter-gate insulation film comprises one of silicon oxide, siliconnitride, aluminum oxide, hafnium oxide, and zirconia oxide, or a stackedfilm of at least two of them.
 11. The nonvolatile semiconductor memorydevice according to claim 1, wherein a plurality of memory cells arearranged and connected to a NAND type.
 12. The nonvolatile semiconductormemory device according to claim 1, wherein a plurality of memory cellsare arranged and connected to an AND type.